This invention relates generally to CATV systems and is more specifically directed to the upstream transmission of data in a two-way CATV system.
Cable television systems typically include a central master source of television programming information, or headend, which imparts programs at different assigned frequencies to a network of cable connections. The network, or cable plant, includes a main distribution cable to which is coupled a plurality of "drop" cables, each of which is coupled to an individual subscriber terminal. Particularly in large metropolitan areas, CATV systems may include one or more master sources which are patched into a common distribution system. CATV signals are broadcast from the headend to individual subscribers in a "downstream" direction over different assigned carrier frequencies typically from 50 to 450 MHz. These downstream signals are in conventional television video signal format and include synchronization, audio, video, etc., components. These downstream signals are transmitted in a branched-out arrangement where all headend initiated signals are provided to all remote subscribers, if authorized.
In a two-way CATV system each subscriber is able to transmit signals back to the CATV headend. These subscriber-initiated signals may include program purchasing requests, opinion poll responses and CATV converter status information. These upstream signals typically make use of out of band frequencies such as in the 5-32.5 MHz band. The upstream signal distribution network is in the form of a "merging tree topology" in which the signals generated by many sources, or subscribers, converge and are transmitted on a single transmission line back to the CATV headend.
In an effort to increase the number of CATV channels available, several types of coherent headends have been devised. One coherent headend is termed Harmonically Related Coherent (HRC), while another is termed Interval Related Coherent (IRC). The HRC headend requires all cable channel picture carriers to be shifted to harmonics of a 6 MHz reference signal. IRC headend channels operate in normally assigned frequencies, except channels 5 and 6, which must be shifted 2 MHz to fall in line with the other channels. In yet another arrangement all cable channel picture carriers are spaced 6 MHz apart from an adjacent channel with the exception of channels 5 and 6. Channels 4 and 5 are separated by less than 6 MHz, while channels 6 and 7 are spaced greater than 6 MHz apart. In all of the aforementioned CATV signal distribution arrangements, essentially all CATV downstream signals are harmonically related.
In general, the main distribution cable is well maintained in a CATV system. However, the connectors coupling either the drop cable to the distribution cable or coupling a subscriber terminal to a drop cable are frequently subject to wear and corrosion. A defective connector produces nonlinearities in the signal handling characteristics of the cable. Under such conditions, a faulty connector operates as a mixing diode in generating sum and difference products of all CATV downstream signals. Only those subscribers having a faulty connector will experience the aforementioned video signal distortion due to the thus generated cross- and intermodulation products. However, these unwanted harmonics are coupled upstream to the CATV headend and in a two-way CATV system seriously degrade upstream communication. Because of the various components of the video signal and side bands associated therewith, the number of modulation products thus produced is very large and substantially contributes to the noise figure across the entire video band. In addition, because television signals employ only downward modulation from a clamped synchronization signal, the effective carrier level of each channel varies according to the video content of the signal. Thus, the various modulation products are a function of video signal content as well as carrier and subcarrier signal frequencies and, as such, present a continually varying source of noise which is extremely difficult to eliminate, or even minimize.
An example of a two-way CATV system utilizing selected frequencies for upstream and downstream communication is disclosed in U.S. Pat. No. 3,943,447 to Shomo. The system described therein takes advantage of a transparent window, in both the upstream and downstream directions, to communications signals over a frequency band from 10-300 KHz wherein the communications signals follow the path of a 60 Hz power signal throughout the system. It is claimed that these communications signals do not interfere with the high frequency television signals being transmitted, nor are they subject to interference by such television signals. This system permits bi-directional communication between any two or more points linked by such a CATV system. The low frequencies of the signals utilized in this system limit its data handling capacity and thus the amount of information which can be transmitted.
The present invention is intended to overcome the limitations of the prior art and solve the aforementioned problems by providing a system which makes use of nonharmonic frequencies for upstream signal transmission in a two-way CATV system.